The invention relates to polymer compositions and the use of these polymer compositions to manufacture semiconductive shields for use in electric cables, electric cables made from these compositions and methods of making electric cables from these compositions. More particularly, the invention relates to composition for use as strippable xe2x80x9csemiconductingxe2x80x9d dielectric shields (also referred to as the core shields, dielectric screen and core screen materials in power cables with cross linked polymeric insulation, primarily with medium voltage cables having a voltage from about 5 kV up to about 100 kV.
In general, semiconducting dielectric shields can be classified into two distinct types, the first type being a type wherein the dielectric shield is securely bonded to the polymeric insulation so that stripping the dielectric shield is only possible by using a cutting tool that removes the dielectric shield along with some of the cable insulation. This type of dielectric shield is preferred by companies that believe that this adhesion minimizes the risk of electric breakdown at the interface of the shield and insulation. The second type of dielectric shield is the xe2x80x9cstrippablexe2x80x9d dielectric shield wherein the dielectric shield has a defined, limited, adhesion to the insulation so that the strippable shield can be peeled cleanly away from the insulation without removing any insulation. Current strippable shield compositions for use over insulation selected from polyethylene, cross-linked polyethylenes, or one of the ethylene copolymer rubbers such as ethylene-propylene rubber (EPR) or ethylene-propylene diene terpolymer (EPDM) are usually based on an ethylene-vinyl acetate (EVA) copolymer base resin rendered conductive with an appropriate type and amount of carbon black. The peel characterization of the strippable shield can be obtained by the proper selection of the EVA with a sufficient vinyl acetate content, usually about 32-40% vinyl acetate, and usually with a nitrile rubber as an adhesion-adjusting additive.
Strippable shield formulations of EVA and nitrile rubbers have been described by Ongchin, U.S. Pat. Nos. 4,286,023 and 4,246,142; Burns et al. EP application Ser. No. 0,420,271 B, Kakizaki et al U.S. Pat. No. 4,412,938 and Janssun, U.S. Pat. No. 4,226,823, each reference being herein incorporated by reference into this application. A problem with these strippable shield formulations of EVA and nitrile rubber is that the EVA""s needed for this formulation have a relatively high vinyl acetate content to achieve the desired adhesion level with the result that the formulations are more rubbery then is desired for high speed extrusion of a commercial electric cable.
Alternative adhesion-adjusting additives have also been proposed for use with EVA, for example waxy aliphatic hydrocarbons (Watanabe et al. U.S. Pat. No. 4,933,107, herein incorporated by reference); low-molecular weight polyethylene (Burns Jr., U.S. Pat. No. 4,150,193 herein incorporated by reference); silicone oils, rubbers and block copolymers that are liquid at room temperature (Taniguchi et al. U.S. Pat. No. 4,493,787 herein incorporated by reference); chlorosulfonated polyethylene, ethylene-propylene rubbers, polychloroprene, styrene-butadiene rubber, natural rubber (all in Janssun) but the only one that appears to have found commercial acceptance was paraffin waxes.